1. Field of the Invention
This concept relates generally to gas recovery plants, otherwise known as “gas liquefaction plants,” and more particularly to a modular gas recovery plant adapted for efficient gas supply, recovery, and storage in an effort to achieve zero helium loss within a closed system.
2. Discussion of Related Art
Although helium (He) is the second most abundant element in the universe, on earth it is scarce and only extracted with difficulty. Helium is found underground, in a gaseous state, as a byproduct of natural radioactive disintegrations. Separation methods are used to isolate helium from other gases found in natural gas wells.
Gas-phase helium is generally transported in containers under high-pressure, while in liquid-phase such helium is generally transported in thermally-insulated containers known as “Dewars” which are adapted to contain helium at or near atmospheric pressure.
Liquid-phase helium is generally obtained from high-power industrial liquefaction plants, referred to herein as “Class XL” liquefiers, which produce liquefied helium in quantities greater than 1000 liters/hour, and require more than 1000 kW of power. These Class XL plants generally yield a liquefaction efficiency of around one liter/hour/kW. Within these Class XL liquefiers, the helium gas undergoes one or more cyclical thermodynamic processes and the gas is then cooled until it reaches its liquefaction temperature. The technology of these liquefaction plants dates from the last century, and can be referred to as “Collins Technology,” examples of which are disclosed in U.S. Pat. Nos. 2,458,894; 3,415,077; and 3,438,220, each issued to Samuel C. Collins.
Moreover, in view of the Class XL liquefiers specified above, various other liquefaction classes may be referred to herein. For example, Class L liquefiers will refer to those liquefiers which produce greater than 100 liters/hour; and Class M liquefiers will refer to those liquefiers which produce greater than 20 liters/hour. All of these large scale liquefiers typically achieve a liquefaction efficiency of about 10 liters/day/kilowatt of input power.
The scientific and industrial applications of helium gas are numerous. In a gas-phase, helium under ambient temperature is useful for welding and as a means for floating balloons. Moreover, in a liquid-phase, helium at atmospheric pressure is generally cold, at or near −269° C., and thus is commonly used for refrigeration of medical and scientific equipment, among other things. Helium is therefore considered to be a valuable resource, lending interest to advancements in helium gas recovery and reuse, especially such recovery and reuse as might be accomplished with negligible or zero loss.
In modern recovery plants, helium gas is generally processed throughout multiple stages, that is, stage 1: recovery; stage 2: storage under pressure; stage 3: purification; stage 4: liquefaction; and stage 5: distribution. These modern plants are generally known to suffer losses in each and every stage as outlined above. Furthermore, even where the loss is very small at one or more stages, when aggregated together the total loss can be significant and often exceeds 10% per cycle. Furthermore, these plants require complex facilities for the storage of vast volumes of highly pressurized gas, regardless of the liquid consumption rate at the particular facility, since the liquefaction rate generally cannot be coordinated, regulated, or adapted for consumption. Finally, without capability to adjust the liquefaction rate, the liquefied helium is produced in volumes that exceed consumption, which necessitates the use of very large storage Dewars, and consequently requires smaller transportation Dewars to distribute the liquid to end users of the liquid gas.
With the advent of closed-cycle refrigerators capable of achieving temperatures below that of liquid helium, such as those based on known Gifford McMahon (GM) and Pulse Tube technologies, liquefiers having lower liquefaction rates and lower maintenance requirements have been developed. In such liquefiers, the gas to be liquefied does not undergo complex thermodynamic cycles, but rather condenses by convection and direct thermal exchange with the different stages of the cryogenic refrigerator and is subsequently stored in a Dewar. However, at present there has yet to be developed such a helium recovery plant based on GM or Pulse Tube technologies which is adapted to yield liquefaction efficiencies comparable to the class XL liquefaction plants described above; that is, one liter/hour/kW.
In an attempt to solve the problem for an individual medical or scientific instrument, that is, providing liquefied helium as needed, liquefaction systems have been developed which incorporate a closed-cycle refrigerator adapted to collect and re-condense helium that is evaporated by the medical or scientific instrument using helium. However, these systems are constructed to use one refrigerator per instrument, and thus underutilize the refrigerator's capacity. For installations in which the direct installation of a refrigerator is technically not feasible, these closed-cycle refrigerator systems do not solve the problem of providing helium as and when required. Moreover, when a large number of instruments require refrigeration, the acquisition and maintenance costs associated with the corresponding number of refrigerators make this solution impractical.
Accordingly, if they were available, gas recovery and purification plants, especially helium-gas recovery and purification plants, based on closed-cycle refrigerator technologies, would be of an immediate and significant interest. With such plants, helium gas being employed as a trace gas in leak-detection processes, or as a cooling medium, could potentially be recovered and reutilized over several cycles with little or no loss, thereby reducing of the need to acquire virgin helium gas. Moreover, the recovery of helium would provide an economic advantage for processes that require pressurized helium gas.